The present invention relates to a system for communication and control, and, more particularly, to a system for communication and control utilizing power lines.
The advantages of being able to transmit and receive communication and control signals over power lines are well-known, and currently a number of devices for communication and control over power lines are commercially available for residential use. The present application uses the term "home communication and control system" to denote such a system of such devices, and uses the term "communication and control signal" to denote a detectable low-energy electrical change which conveys information for measurement or command. The present application uses the term "power main" to denote a primary circuit carrying electrical power from the utility, and uses "local power line" to denote a tributary electrical power circuit, either new or existing, which serves to connect one or more independent loads to a power main. The present application uses the term "leg" to denote a branch of an electrical power circuit. The present application uses the term "sensor" to denote any device whose electrical characteristics vary in a known manner according to the physical and/or chemical state of its environment. The purpose of sensors is to enable the measurement of external quantities in electrical terms. Examples of sensors include, but are not limited to, transducers of temperature, light, humidity, magnetic field, pressure, fluid flow, sound, pH, smoke density, radioactivity, and other physically measurable quantities. The electrical characteristics of sensors which vary include, but are not limited to, resistance, frequency, voltage, current, and other such electrical properties. The communication and control effected by a home communication and control system is sensing of one or more conditions, and in response to those conditions, the control of one or more electrical loads, which include, but are not limited to apparatus such as water heaters, lighting, space heaters, ventilators, and air conditioners, humidifiers, dehumidifiers, home appliances, security systems, and the like. The present application uses the term "load" to denote an electrical load.
The principal advantages of home communication and control systems are those of convenience, flexibility, and low installation cost. First of all, no additional wiring is needed for communication and control of lighting, heating, ventilating, and air conditioning, home appliances, security systems, and the like. Thus, the devices can easily and inexpensively be retrofit into older homes. Secondly, the component devices of home communication and control systems can be conveniently located anywhere there is an electrical power main, to provide more design and layout options, and they may be moved from place to place as needed. Home communication and control systems offering continuous communication over power lines include those available from National Semiconductor (LM1893/2893), SGS Thomson (ST7537), Philips (TDA5051), and also from other semiconductor and consumer electronics manufacturers.
Furthermore, widespread interest in transmitting information over power lines is seen in the international standards which have been established for this, including the European CENELEC EN 50065 standard for power line communication.
Unfortunately, home communication and control systems suffer from performance problems related to the use of a power main to transmit signal-level energies. First, a power main represents a very low impedance signal sink, typically several Ohms, and this causes a serious loss of signal strength between the transmitter and receiver circuits. Another problem is related to the high wideband noise present on power mains. This noise can interfere with intended communications and cause spurious signals to be received. Still another problem arises from conflicts between similar systems in nearby locations which share a power main. These problems adversely affect the reliability of home communication and control systems and diminish their value, even in non-critical residential applications. Additional disadvantages of present-day home communication and control systems include their functional complexity and relatively high price of their component devices.
Many different configurations of communication circuits utilizing power lines have been made. For example, U.S. Pat. No. 3,500,132 to Garrett (hereinafter referred to as "Garrett") discloses a scheme for utilizing a single pair of electrical lines to transmit both power and information by interrupting the power according to the information to be transmitted. U.S. Pat. No. 3,909,821 to Jagoda et al. (hereinafter referred to as "Jagoda") discloses a scheme for transmitting audio information over power lines by utilizing filters to reject the 60 Hz power frequency. U.S. Pat. No. 4,031,528 to Harrison (hereinafter referred to as "Harrison") discloses a transponder for sending digital data over power lines by encoding the information in a unipolar potential over the power lines, whose frequency is a multiple of the power line frequency. U.S. Pat. No. 4,348,582 to Budek (hereinafter referred to as "Budek") discloses apparatus for transmitting digital information over power lines by causing a brief near short-circuit condition to occur, and apparatus for receiving this information by sensing the loss of voltage. U.S. Pat. No. 4,642,637 to Baer (hereinafter referred to as "Baer") discloses a method for transmitting data over power lines by switching in a load to generate current signals representing the data.
All of the innovations mentioned above are concerned with methods of representing information so that it may be transmitted over power lines. With the exception of Garrett, all of them can utilize existing power lines to transmit information. Garrett, however, discloses an arrangement whereby the "power lines" are usable only to supply power to the transmitting device, and cannot be used, for example, to power an independent load. None of these prior innovations, moreover, address the general problem of overcoming the problems of noise and low impedance inherent in power lines because of their connection to the power main.
There is thus a need for, and it would be highly advantageous to have, a system which would overcome the above-mentioned disadvantages of presently-available home communication and control systems for communication and control over power lines. This goal is met by the present invention.